Atypical Mycobacterial Infection

Non-tuberculous mycobacteria (NTM), also called atypical mycobacteria or mycobacteria other than tuberculosis (MOTT), are a diverse group of acid-fast bacilli found ubiquitously in the environment. They cause a spectrum of disease from localised skin and soft-tissue infection to disseminated disease in immunocompromised hosts.

How they are acquired. Environmental acquisition predominates, especially from water and soil, and most musculoskeletal infection follows environmental inoculation rather than routine person-to-person spread. Respiratory isolation is not required for localised musculoskeletal disease. Incidence has been increasing worldwide over past decades, and the risk factors are immunosuppression, aquatic exposure, fish handling and tropical travel.

Which organisms. M. marinum is the most common NTM causing hand infections, and MAC the most common NTM overall in the immunocompromised. Rapid growers are increasingly recognised in surgical site infections.

What the orthopaedic surgeon sees. Often a culture-negative chronic infection, in one of these forms:

• Tenosynovitis, especially of the hand and wrist flexor sheaths
• Septic arthritis, with a chronic, indolent presentation
• Osteomyelitis, less common and usually adjacent to soft-tissue infection
• Post-surgical infection from rapid growers after procedures
• Bursitis, olecranon and prepatellar

Why it is missed. NTM infections are commonly missed because of their indolent nature, because standard bacterial cultures do not detect them, and because AFB cultures must be specifically requested. The exam tests recognition, appropriate investigation and an understanding of prolonged multi-drug therapy. Think of NTM in:

• chronic indolent tenosynovitis
• culture-negative joint infection
• a chronic wound with granulomas
• a chronic indolent infection, especially in the hand, that fails standard antibiotics and has negative routine cultures
• an immunocompromised host
• aquarium or fish exposure
• a post-procedural wound infection

Runyon classification. The traditional classification groups NTM by growth rate and pigment production. It is still tested and helps predict clinical syndromes and guide initial therapy, although molecular methods now provide definitive identification.

Slow against rapid. The growth-rate split carries clinical meaning. Slow growers favour environmental exposure, and granulomatous histology is typical of them. Rapid growers often follow healthcare exposure (post-surgical wounds, injections, catheters), have variable resistance patterns and are often resistant to multiple agents, which makes treatment more challenging; both groups need species-directed therapy.

MAC. M. avium and M. intracellulare are clinically indistinguishable. MAC is highly resistant and needs prolonged multi-drug therapy.

Why M. marinum prefers the cold. M. marinum is a temperature-restricted mycobacterium: it grows best at 30-32°C and poorly at core body temperature (37°C), the same cool-loving biology as M. ulcerans and M. leprae. It therefore preferentially infects the cooler, superficial, distal tissues, the skin and the hands, wrists and extremities, which is exactly why the classic presentation is hand or wrist tenosynovitis with skin nodules rather than deep visceral disease.

It also explains why, in an immunocompetent host, M. marinum seldom spreads to warm deep or visceral sites; dissemination is essentially confined to significant immunocompromise. The same biology dictates how the laboratory must grow it.

Two arms. Deep musculoskeletal infection usually needs both surgical source control and prolonged species-directed antimicrobial therapy.

PrinciplesM. marinumRapid GrowersMAC and Others

Surgery. Debridement is required for most NTM musculoskeletal infections: tenosynovectomy for flexor sheath involvement, debridement of infected and necrotic tissue, and removal of rice bodies and thickened synovium, with multiple tissue samples sent for culture. Extensive disease may require staged procedures.

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Antimicrobials. The regimen is species-, site- and susceptibility-directed, and susceptibility testing guides the specific choices. Skin-limited and deep disease require different drug numbers and durations: deep tendon, joint, bone or implant disease usually requires multiple active drugs for months. A macrolide counts as active only when phenotypic and molecular susceptibility support it. Monitor for drug toxicity (hepatic, auditory, visual).

Protect Active Drugs

Do not add an unprotected macrolide to an inadequate regimen. Skin-limited M. marinum may occasionally be treated with one proven active agent, but deep tendon, joint, bone, implant, disseminated MAC and rapid-grower disease generally require multiple active drugs selected by species, molecular resistance markers and susceptibility testing.

Mnemonic

COMBONTM Treatment Principles

C

Culture before committing

Obtain multiple deep tissue samples before antimicrobials whenever the patient is stable

O

Organism-directed therapy

Species, subspecies, site and susceptibility determine the regimen

M

Multiple active drugs for deep disease

Protect the macrolide and treat for months when tendon, joint, bone or implant is involved

B

Base on susceptibility testing

Check inducible and acquired macrolide resistance in rapid growers

O

Operate for source control

Debride necrotic tissue, excise infected synovium and remove loose implants when required

Hook:COMBO means culture first, identify the organism, use enough active drugs for the disease depth, and obtain surgical source control.

Drugs. Deep M. marinum disease is usually treated with at least 2 active agents, continued beyond clinical resolution.

Clarithromycin 500mg BD + Rifampicin 600mg daily

Duration

3-6 months

Notes

Most commonly used regimen, check drug interactions with rifampicin

Clarithromycin 500mg BD + Ethambutol 15mg/kg

Duration

3-6 months

Notes

Alternative if rifampicin contraindicated

Doxycycline 100mg BD + Rifampicin 600mg daily

Duration

3-6 months

Notes

If macrolide-intolerant

Trimethoprim-sulfamethoxazole DS BD + Rifampicin

Duration

3-6 months

Notes

Alternative regimen

First-Line Regimens for M. marinum

Regimen	Duration	Notes
Clarithromycin 500mg BD + Rifampicin 600mg daily	3-6 months	Most commonly used regimen, check drug interactions with rifampicin
Clarithromycin 500mg BD + Ethambutol 15mg/kg	3-6 months	Alternative if rifampicin contraindicated
Doxycycline 100mg BD + Rifampicin 600mg daily	3-6 months	If macrolide-intolerant
Trimethoprim-sulfamethoxazole DS BD + Rifampicin	3-6 months	Alternative regimen

Treatment Duration

For skin/soft tissue M. marinum: treat for 1-2 months after clinical resolution (typically 3-6 months total; roughly 3-4 months when skin-limited). For tenosynovitis with surgical debridement: 3-6 months minimum. For osteomyelitis: 6-12 months. Continue until clinical cure, then add safety margin.

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Surgery. Beyond tenosynovectomy, release the carpal tunnel if the median nerve is compressed. Multiple staged debridements may be needed, and the wound is left open or loosely closed if contamination is significant.

Timing. Consider early therapy if the diagnosis is suspected pre-operatively, although in a stable patient deep tissue samples come before antimicrobials. Early diagnosis and treatment improve outcomes significantly.

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M. fortuitum

Susceptibility Pattern

Often susceptible to amikacin, fluoroquinolones, doxycycline, trimethoprim-sulfamethoxazole or imipenem, but the isolate must be tested

Recommended Regimen

Use at least 2 proven active drugs for deep disease; do not assume macrolide susceptibility

M. chelonae

Susceptibility Pattern

Macrolides may be active; tobramycin is often more active than amikacin, with marked isolate variation

Recommended Regimen

Use a macrolide only if susceptible and add another active agent for deep disease

M. abscessus complex

Susceptibility Pattern

erm(41) and rrl determine whether a macrolide can be counted as active; resistance is common

Recommended Regimen

Expert-directed initial phase with at least 3 active drugs, usually including IV agents, followed by a susceptibility-matched continuation phase

Treatment of Rapid Growers

Organism	Susceptibility Pattern	Recommended Regimen
M. fortuitum	Often susceptible to amikacin, fluoroquinolones, doxycycline, trimethoprim-sulfamethoxazole or imipenem, but the isolate must be tested	Use at least 2 proven active drugs for deep disease; do not assume macrolide susceptibility
M. chelonae	Macrolides may be active; tobramycin is often more active than amikacin, with marked isolate variation	Use a macrolide only if susceptible and add another active agent for deep disease
M. abscessus complex	erm(41) and rrl determine whether a macrolide can be counted as active; resistance is common	Expert-directed initial phase with at least 3 active drugs, usually including IV agents, followed by a susceptibility-matched continuation phase

M. abscessus - Most Difficult to Treat

M. abscessus complex requires subspecies identification plus erm(41), rrl and phenotypic susceptibility assessment. A macrolide may be retained for immunomodulatory effect when inducible resistance is present, but it must not be counted as an active drug. Deep disease usually needs surgical source control and an expert-selected initial regimen containing at least three active agents.

Treatment should be guided by infectious disease specialist consultation.

M. avium complex (MAC)

Treatment Regimen

Clarithromycin + ethambutol + rifampicin (or rifabutin in HIV)

Duration

12-18 months (until culture negative for 12 months)

M. kansasii

Treatment Regimen

Isoniazid + rifampicin + ethambutol (similar to TB)

Duration

12-18 months (until culture negative for 12 months)

M. ulcerans (Buruli ulcer)

Treatment Regimen

Rifampicin + clarithromycin

Duration

8 weeks, surgery for large lesions

Treatment of Other NTM Species

Organism	Treatment Regimen	Duration
M. avium complex (MAC)	Clarithromycin + ethambutol + rifampicin (or rifabutin in HIV)	12-18 months (until culture negative for 12 months)
M. kansasii	Isoniazid + rifampicin + ethambutol (similar to TB)	12-18 months (until culture negative for 12 months)
M. ulcerans (Buruli ulcer)	Rifampicin + clarithromycin	8 weeks, surgery for large lesions

MAC. The backbone is a macrolide plus ethambutol, with companion drugs selected for site, burden and susceptibility, and the three-drug regimen reduces resistance. Monitor for macrolide toxicity and rifampicin interactions. The 12-18-month duration in the table comes from pulmonary MAC, continued until sputum cultures have been negative for 12 months. Disseminated MAC in AIDS may need lifelong suppressive therapy if immune reconstitution is not achieved.

M. kansasii. The most susceptible of the slow-growing NTM, more so than MAC, it responds well to rifampicin-based regimens, with susceptibility and disease site guiding the companion drugs and duration. Treatment is similar to TB but longer, it is usually curable with medical therapy alone, and relapse is less common than with MAC.

Prolonged treatment is common, but the drug number, regimen and endpoint must be matched to the organism, disease site, source control and response.

TNF-alpha holds granulomas together. Tumour necrosis factor-alpha is essential for both the formation and the maintenance of the granulomas that wall off mycobacteria, so biologic immunosuppression changes both susceptibility and presentation. Blocking it lets contained or newly acquired mycobacteria proliferate.

TNF inhibitors raise NTM (and TB) risk. The monoclonal anti-TNF agents (infliximab, adalimumab, golimumab, certolizumab) carry a higher granulomatous-infection risk than the soluble receptor (etanercept). NTM disease, pulmonary and extrapulmonary, including tenosynovitis and skin and soft-tissue infection, is a recognised and rising complication of biologic therapy for rheumatoid arthritis, inflammatory bowel disease and psoriasis.

In practice. In a patient on a biologic (or other immunosuppression) with an indolent, atypical or culture-negative musculoskeletal infection, actively pursue NTM. Consider holding the biologic during treatment, and involve infectious diseases and rheumatology. Tuberculosis is screened for before starting a biologic; NTM warrants the same vigilance during therapy.

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Global Epidemiology

NTM disease incidence has risen worldwide over recent decades, attributed to genuine increase, improved molecular recognition, an ageing population and expanding use of immunosuppressive/biologic therapy. Musculoskeletal disease is usually acquired from soil, water or healthcare exposure; routine respiratory isolation is not required for localized NTM infection. Species distribution is geographically patterned:

• MAC dominates slow-grower disease in North America, Western Europe and East Asia.
• M. kansasii clusters around specific urban/industrial water supplies (historically central USA, UK, parts of Europe and mining regions).
• M. abscessus and other rapid growers are proportionally more common in East/Southeast Asia and are the leading healthcare-associated NTM globally.
• M. ulcerans (Buruli ulcer) is endemic in West/Central Africa, coastal south-eastern Australia and parts of South America and the Western Pacific - consider it in any traveller or resident from an endemic zone with a painless, progressive skin ulcer.

Side-by-Side Guideline Comparison

There is broad cross-society consensus on the fundamentals: secure adequate specimens, confirm the species or subspecies, test relevant susceptibility and resistance markers, protect active drugs from functional monotherapy, combine prolonged therapy with source-control surgery for deep disease, and follow clinical, imaging and microbiological response. Regimen detail differs substantially by organism and syndrome.

Laboratory and Registry Notes

• NTM is not a notifiable disease in most jurisdictions (in contrast to TB), so true incidence is under-captured and surveillance relies on voluntary laboratory reporting and national reference-laboratory networks.
• Definitive species/subspecies identification and susceptibility testing are concentrated in reference laboratories worldwide (e.g. national mycobacterial reference centres in the UK, France, Germany, USA, Korea, Australia). Always flag clinical suspicion - including the need for 30-32°C incubation when M. marinum is suspected.
• There is no dedicated implant/arthroplasty-style registry for NTM; the evidence base is case series, reference-laboratory cohorts and the society guidelines above.

High- vs Limited-Resource Practice Variation

• High-resource settings: molecular species/subspecies identification (line-probe assays, 16S/hsp65 sequencing, MALDI-TOF), erm(41) testing, drug-level monitoring, outpatient/home IV therapy and multidisciplinary ID input enable tailored, susceptibility-guided regimens.
• Limited-resource settings: prolonged AFB culture and molecular testing may be unavailable, so diagnosis leans on clinical pattern plus granulomatous histology, and empiric clarithromycin-based combinations are common. For Buruli ulcer, WHO-endorsed oral rifampicin + clarithromycin has shifted care from inpatient injectable regimens and extensive surgery toward community-based oral therapy, markedly improving access in endemic regions.

Related pages: Tuberculosis of the Spine (Pott's Disease) reinforces that rice bodies and granulomas never separate tuberculosis from NTM; culture and molecular identification are decisive. Flexor Tenosynovitis contrasts the acute pyogenic syndrome with indolent granulomatous tenosynovitis. Septic Arthritis Pathophysiology and Osteomyelitis Pathophysiology cover the joint and bone consequences of delayed source control.

Optimal antibiotic duration. No randomised trials define duration for musculoskeletal NTM. Practice ranges from 1-2 months beyond clinical resolution for skin-limited M. marinum to 6-12 months or more for tenosynovial and osseous disease, and most regimens are extrapolated from pulmonary guidelines and expert opinion rather than dedicated bone and joint evidence.

Surgery against antibiotics alone. Whether all rice-body tenosynovitis needs synovectomy, or whether early-recognised superficial M. marinum can be cured medically, is debated. In Aubry's series 52% of patients were managed without surgery, and failure related to deep-structure involvement rather than to the antibiotic regimen. The threshold for, and extent of, debridement is individualised.

erm(41) and macrolide reliance. Clarithromycin is the "cornerstone", yet inducible erm(41) resistance makes it unreliable for true M. abscessus despite favourable in-vitro MICs. How heavily to weight (and how routinely to test) inducible resistance, and the role of newer agents (e.g. amikacin liposome inhalation, clofazimine, tedizolid, bedaquiline), remains unsettled.

Empiric therapy while awaiting culture. Because AFB culture takes 2-6 weeks, clinicians must decide whether to start empiric multidrug therapy on clinical or histological suspicion or wait for confirmation. Premature single-agent or steroid use can worsen disease and obscure diagnosis; over-eager empiric therapy risks toxicity and culture suppression.

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